As far as Android smartphones are concerned, Samsung is still pretty infamous for its own software slowing down even the company's flagship offerings in the past. According to reports from multiple users, the security update from June is further adding to this problem by significantly slowing down the Galaxy S9+.
The good news is that the lag in performance can be addressed by cache partition clearing and optimization tools, while the bad news is that such fixes seem to have only temporary effect. After a while, the smartphones are reverting back to being laggy. The only solution as of now is to uninstall the June security patch and erase the entire device, but that's a lot of hassle, which is not advised unless you know how to install ROMs with Odin 3.
Even for the average Joe, the problem might disappear pretty soon though, because the July security patch has already started rolling out to the Galaxy S9 and S9+ in many regions. Hopefully, it will be able to address this issue effectively.
Check out our firmware section to find all the latest updates for your Samsung smartphones and tablets.