How much do you want to spend? You can get 8GB memory and a 1TB SSD for about $120. If you have 16 GB installed already you can't add more memory according to the specs from Dell I read. If you have 8 GB or less installed, another 8 may provide a boost for some functions. If you have only 4 GB now, then replacing that with 16 GB will definitely be a noticeable improvement. $28 at Newegg for an 8 GB module, $43 for a pair. The SSD will help a bit more, depending on how you use the machine. Hard drive related actions will speed up significantly, so booting for example will take a lot less time. If you have only 4 GB of memory installed then your system is probably using your hard drive to cache info it needs when executing certain programs. That will slow everything down significantly. If this is the case, then the upgrades should make you happier.
Also, if your existing HD is full that will slow the system down as well. In this case get a larger capacity SSD than you already have installed. I find it amazing how quickly Windows can fill up a hard drive so the bigger the better. Like I said, I found a 1 TB SSD at Newegg for $90. If you look around you may find an even better deal.
These upgrades will not help you with the Windows 11 incompatibility though.