Adult sites are just web sites, the scary FUD that some claim to be tied to them is mostly just misleading lies made by those pushing their own bias and agendas. Yeah some adult sites will flood your browser with all kinds of junk and cruft but so do a lot of non-adult sites. A big culprit are the stable of coupon sites that those looking for shopping discounts rely upon. Major news sites, like the Chicago Tribune site as just one example, are densely packed with ads that insert dozens of tracking cookies into your browser with each page you view (dozens not being an exaggeration in any way). So don't assume that your typical adult site is going to be more or less prone to adding any kind of system resource drain service into your phone, you need to be wary of almost everything you do online.
Use Incognito Mode to automatically wipe cookies and cache when you quit each session. Or use a privacy and security focused browser app like Firefox Focus or Brave that will do the same by default.