Your DDR4 RAM options are: 8, 16, or 32 GB PC4-19200 (2400 MHz) when you have a Core i3 CPU or PC4-21300 (2666 MHz) when you have a Core i5/i7. It is also possible to go to 64GB with either of the Core i5/i7 CPU's. The limitation is the CPU's clocking.
As to speed I think you've got things backwards the higher end models have higher access speed RAM What are Memory Timings & How they Work: CAS, RAS and tRAS Explained
Unlike PC, Mac's timings are fixed based on the CPU ID held in the systems EFI.
The quantity of RAM does have a bearing! The deeper the reach the longer it takes! Independent of the RAM's clocking. Its advised not to max out the RAM if you don't need it! So make sure the applications and your data set can really use it beforehand. 32GB is a good size for an advanced amateur. The 21.5" system is not seen as a Pro level machine and do its graphics it is really a lightweight for what a Pro would need.
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