Method:
I clocked the time it takes to cast 100 spells and that’s pretty much it. I started by putting all points in dex at creation, get aug dex 5 etc to see the what i got with the highest possible dex, then i removed items and clocked a new time. If the new time was different from the first one, i adjusted items, starting points and RAs and tested a number between the two. I then repeated this until i found the “breaking point.

Not all tests were done with 100 casts as i didn’t have enough power. The spread heal (3.5s) for example was tested with 25 casts and then multiplied by 4.

Result:
1) There are “breaking points” where your cast speed gets significantly faster.
2) Anything in between these breaking points gives very little or no return.
3) Maybe someone will, but i fail to see a pattern in these breaking points.
4) The breaking points seem to apply to all spells with the same cast time, but i have not and am not going to test it for all spells.

+10% CAST SPEED DURING THE WHOLE TEST

Breaking points:
2.0s —> 392 dex, 344 dex, 337 dex
2.1s —> 356 dex, 354 dex, 308 dex
2.2s —> 374 dex, 368 dex, 320 dex
2.3s —> 386 dex, 380 dex, 332 dex
2.4s —> 392 dex, 350 dex, 307 dex
2.5s —> 362 dex, 319 dex
2.6s —> 374 dex, 332 dex
2.7s —> 386 dex, 344 dex
2.8s —> 392 dex, 356 dex, 320 dex
2.9s —> 368 dex, 362 dex, 332 dex
3.0s —> 374 dex, 338 dex
3.5s —> 356 dex, 326 dex

The test:

(2.0s delve) Tested with Mentalist nearsight
407 dex = 95.4 sec
395 dex = 96.0 sec
392 dex = 95.6 sec
—————————- Breaking point —————————-
391 dex = 111.0 sec
390 dex = 110.0 sec
388 dex = 110.8 sec
374 dex = 110.6 sec
360 dex = 110.9 sec
355 dex = 110.8 sec
346 dex = 111.0 sec
344 dex = 110.9 sec
—————————- Breaking point —————————-
343 dex = 118.2 sec
341 dex = 117.8 sec
338 dex = 118.3 sec
—————————- Breaking point —————————-
337 dex = 125.6 sec
318 dex = 126.4 sec

(2.1 sec) Tested with 2.1s greater heal
382 dex = 94.9 sec
357 dex = 96.4 sec
356 dex = 96.8 sec
—————————- Breaking point —————————-
355 dex = 102.4 sec
354 dex = 102.9 sec
—————————- Breaking point —————————-
353 dex = 107.3 sec
352 dex = 107.4 sec
350 dex = 108.0 sec
342 dex = 110.4 sec
326 dex = 110.0 sec
311 dex = 110.8 sec
309 dex = 111.2 sec
308 dex = 110.6 sec
—————————- Breaking point —————————-
307 dex = 119.6 sec
306 dex = 118.9 sec

(2.2 sec) Tested with 2.2s greater heal
382 dex = 95.2 sec
375 dex = 95.2 sec
374 dex = 94.8 sec
—————————- Breaking point —————————-
373 dex = 99.4 sec
371 dex = 99.5 sec
368 dex = 99.6 sec
—————————- Breaking point —————————-
367 dex = 110.3 sec
365 dex = 109.2 sec
364 dex = 110.2 sec
362 dex = 111.0 sec
352 dex = 110.7 sec
346 dex = 111.2 sec
339 dex = 110.4 sec
326 dex = 111.6 sec
321 dex = 112.8 sec
320 dex = 113.2 sec
—————————- Breaking point —————————-
319 dex = 125.5 sec
316 dex = 125.0 sec
311 dex = 126.2 sec

(2.3 sec) Tested with 2.3s greater heal
391 dex = 94.6 sec
388 dex = 94.4 sec
386 dex = 94.3 sec
—————————- Breaking point —————————-
385 dex = 99.8 sec
384 dex = 98.9 sec
380 dex = 98.7 sec
—————————- Breaking point —————————-
379 dex = 109.7 sec
378 dex = 109.6 sec
363 dex = 110.0 sec
351 dex = 110.8 sec
338 dex = 110.0 sec
332 dex = 111.2 sec
—————————- Breaking point —————————-
331 dex = 125.9 sec
330 dex = 126.1 sec
329 dex = 125.7 sec
326 dex = 126.0 sec

(2.4 sec) Tested with 2.4s greater heal
392 dex = 96.2
—————————- Breaking point —————————-
391 dex = 109.8 sec
386 dex = 110.8 sec
361 dex = 110.2 sec
358 dex = 110.9 sec
353 dex = 110.8 sec
351 dex = 111.7 sec
350 dex = 111.0 sec
—————————- Breaking point —————————-
349 dex = 126.1 sec
336 dex = 126.2 sec
325 dex = 126.2 sec
313 dex = 126.4 sec
308 dex = 126.3 sec
—————————- Breaking point —————————-
307 dex = 140.0 sec
306 dex = 142.6 sec
304 dex = 143.0 sec
299 dex = 142.2 sec

(2.5 sec) Tested with animist lifetap and 2.5s heal
402 dex = 110.4 sec
394 dex = 110.7 sec
380 dex = 110.9 sec
373 dex = 111.3 sec
367 dex = 111.4 sec
362 dex = 111.4 sec
—————————- Breaking point —————————-
361 dex = 123.9 sec
360 dex = 124.0 sec
359 dex = 123.6 sec
356 dex = 123.8 sec
335 dex = 126.6 sec
320 dex = 127.6 sec
—————————- Breaking point —————————-
319 dex = 140.4 sec
315 dex = 141.0 sec
310 dex = 142.6 sec
295 dex = 142.2 sec

(2.6 sec) Tested with hib baseline nuke and 2.6s major heal
424 dex = 110.5 sec
385 dex = 110.4 sec
380 dex = 110.8 sec
377 dex = 111.5 sec
375 dex = 111.6 sec
374 dex = 111.3 sec
—————————- Breaking point —————————-
373 dex = 122.6 sec
340 dex = 126.0 sec
333 dex = 128.6 sec
332 dex = 129.8 sec
—————————- Breaking point —————————-
331 dex = 141.6 sec
330 dex = 141.4 sec
326 dex = 140.8 sec
319 dex = 141.2 sec

(2.7 sec) Tested with 2.7s major heal
392 dex = 108.6 sec
391 dex = 110.3 sec
388 dex = 110.4 sec
386 dex = 110.2 sec
—————————- Breaking point —————————-
385 dex = 119.1 sec
384 dex = 121.0 sec
357 dex = 125.4 sec
350 dex = 126.3 sec
345 dex = 128.1 sec
344 dex = 126.9 sec
—————————- Breaking point —————————-
343 dex = 140.8 sec
342 dex = 141.3 sec
340 dex = 140.9 sec
336 dex = 141.2 sec
333 dex = 142.0 sec

(2.8 sec) Tested with menta spec nuke, animist ST bomb and 2.8s major heal
405 dex = 110.4 sec
402 dex = 110.6 sec
398 dex = 110.8 sec
394 dex = 111.3 sec
392 dex = 111.5 sec
————————— Breaking point —————————
391 dex = 125.6 sec
390 dex = 126.2 sec
388 dex = 126.1 sec
386 dex = 126.5 sec
380 dex = 126.5 sec
379 dex = 127.2 sec
375 dex = 127.1 sec
370 dex = 126.8 sec
365 dex = 126.7 sec
360 dex = 127.0 sec
356 dex = 127.1 sec
————————— Breaking point —————————
355 dex = 141.2 sec
340 dex = 142.3 sec
333 dex = 142.4 sec
326 dex = 142.0 sec
324 dex = 142.1 sec
321 dex = 142.8 sec
320 dex = 143.1 sec
————————— Breaking point —————————
319 dex = 154.0 sec
318 dex = 153.2 sec

(2.9 sec) Tested with 2.9s major heal
382 dex = 126.0 sec
368 dex = 126.8 sec
————————— Breaking point —————————
367 dex = 135.2 sec
366 dex = 136.6 sec
364 dex = 136.2 sec
362 dex = 136.3 sec
————————— Breaking point —————————
361 dex = 141.2 sec
354 dex = 142.3 sec
336 dex = 141.6 sec
334 dex = 141.2 sec
332 dex = 142.5 sec
————————— Breaking point —————————
331 dex = 154.3 sec
330 dex = 154.4 sec
323 dex = 158.4 sec

(3.0s delve) Tested with menta confusion and 3.0s major heal
424 dex = 126.0 sec
403 dex = 126.8 sec
384 dex = 127.2 sec
380 dex = 127.0 sec
375 dex = 128.0 sec
374 dex = 127.6 sec
—————————- Breaking point —————————-
373 dex = 141.6 sec
369 dex = 142.3 sec
346 dex = 144.0 sec
339 dex = 147.1 sec
338 dex = 149.8 sec
—————————- Breaking point —————————-
337 dex = 158.4 sec
335 dex = 158.0 sec
322 dex = 158.4 sec

(3.5 sec) Tested with lvl 30 spreadheal
382 dex = 156.4 sec
370 dex = 157.2 sec
365 dex = 158.4 sec
360 dex = 157.9 sec
358 dex = 158.0 sec
356 dex = 157.7 sec
—————————- Breaking point —————————-
355 dex = 171.5 sec
354 dex = 173.6 sec
326 dex = 173,2 sec
—————————- Breaking point —————————-
325 dex = 188.0 sec
324 dex = 188.5 sec
321 dex = 189.5 sec
316 dex = 188.7 sec
301 dex = 190.5 sec