1.15 was a braindead update. No api changes and no major mechanics. Additionally, the base cause of the lag isn’t any minecraft version. It’s some underlying hardware/system issue.
1.15 was made to fix issues that 1.14 brought, however, I do think updating to 1.14 itself wasn’t an issue, just poor coding on the plugins and not being properly maintained made it worse when we went 1.14.
