Was the Pentium D that slow?
It is. That machine is a slug by today's standards. It gets crippled by everything now. Youtube is pretty much out on it for certain resolutions/framerates. For example, it can't really cope with 1080p playback, or any mode that uses 60fps, it just can't keep up. I had to install a browser plugin to block 60fps playback so I could watch the video in 720p. Local videos are fine, it's just the player Youtube uses that's unusable on it. If all else fails, 480p becomes the optimal playback resolution since the CPU is hit very hard with Youtube.
Games also have to use very low settings to be playable, even not very demanding ones that are a bit old now. TF2 for example on super low settings (including using an autoexec with engine variables to further drop it beyond the options screen) results in an average of 15fps on 720p. That's what I was used to for that machine, stuff being barely playable. Of course that doesn't apply to old titles, just newer ones after 2007 or so.
Further, that machine takes ages to do literally anything. Applying Windows updates usually takes 30 to 60 minutes alone (Win7 32-bit by the way, probably not the best choice but it's what I got). The machine would greatly benefit from an SSD (it currently uses a 7200RPM HDD), though the Pentium D will still bottleneck everything.
The modern internet is just usable with a late NetBurst CPU. I'm running a dual 3.6 GHz Xeon rig from 2005 with 12 GB and a GT 730. Obviously, modern games are out of the question but it can handle 1080p video just fine. It struggles with higher resolutions though.
Yeah 1080p is definitely the limit, even then that's pushing it in some cases. Games from 10 years ago for me have to be run in 720p, 1080p is just not feasible unless you enjoy playing on low settings at 15fps. It's not a bad platform for some retro gaming for its time period, but honestly you'd probably be better off with a proper Core 2 Duo instead.
Pentium MMX 233 | 64MB | FIC PA-2013 | Matrox Mystique 220 | SB Pro 2 | Music Quest MPU Clone | Windows 98SE
MT-32 | SC-55mkII, 88Pro, 8820 | SB16 CT2230
3DFX Voodoo 1&2 | S3 ViRGE GX2 | PowerVR PCX1&2 | Rendition Vérité V1000 | ATI 3D Rage Pro