slow SSD

Soulfood, yes SSDs can read fast but I'm not sure what you mean by delete slow. Almost any hard drive, SSD or mechanical can read faster than it can write. Yes, in most cases a larger size will be faster, they can put more nand in it. But, the advertised speed will make little difference in an OS drive. For an OS drive it is the 4kb reads and response times that will matter most. Even a smaller SSD will have a response time of 0.1ms (ms=1,000 of a second). That is what makes them so great as OS drives. The normal mechanical hard drive has a response time of around 15.0ms. The only real numbers that matter for the majority of OS users is rsponse time and 4kb read time. You will notice no difference in write time as you do very few writes to an OS drive.

Slow is a relative term. They'll delete data slower than they read it, but still pretty fast compared to a mechanical drive. With SSDs, the actual deletion process is taken care of behind the scenes by TRIM or the drives proprietary garbage collection.

Bigger drives of the same type/generation/controller are typically faster than smaller ones due to the way the memory blocks are allocated.

You need not worry about the deletes. When you click "delete", a bit is being flipped in the MFT to mark the space as "available". And for SSDs, also a Trim signal is being sent to the controller. Then, as Steve says, he actual Trim deletion process is run asynchronously in the background without the OS even knowing.

In SSDs or operating systems that do not support Trim, this deletion process takes place ahead of each write into that space. That makes write operations twice as long when Trim is not available.