Lets me bring some tidbits to your discussion of building EPG by DVR/STB. My base is own experience in writing programs [made DLLs for TSreader] for parsing SI tables [PSIP].
Before you'll start beating me - yes, the DLLs are working on PC under Windows OS, I'll tell you - the DVR/STB have multicore chip with HW decoders of video/audio compressed streams. Perhaps low end, because each STB mfg want squeeze each penny out of each component the device. Economics !
So, for good grasp of the theme, we will accept similarity between those two [PC and DVR].
Now, the stream - TS. Transport stream.
Some obvious info about it: one TS aka a mux does reside on one RF channel (transponder for sats) and contain PIDs, eg separated sub-streams what are carry system info [PSIP] and data (time stamps, UDP for some, pictures for audio channels, etc.) and compressed video/audio for "channels".
PSIP consist many deferent tables. You could find more info in A-53 standard.
Here is different point for your thoughts - DVR OS simultaneously
processing one current mux (or two, if there are two physical OTA tuners, or four ..) . It will decompress A/V packets of that channel(s) what you're watching/recording and parse PSIP tables same time. Question is how powerful CPU/cores to do that simultaneously ?
If there is a bottleneck, then FW developers using prioritizing mechanism and a scheduler/manager to control the tasks. But it should be in that time ranges/limits what define PSIP rotation and allow deliver smooth video/clear audio for viewers.
As for my DLLs, I have at my disposal pretty powerful PCs with multi core CPU supporting multithreading, so I didn't experienced problems with watching channel by VLC and parsing many PSIP/PIDs/tables as I'm wish.
PS. For curious minds, we could arrange simple project - I'll provide you new DLL what would give you stats of PSIP data as a compliment of TSreader parser of PSIP tables. From your side it will require a PC with Windows + OTA tuner [USB stick is OK, check compatibility with TSreader] + TSreaderLite + my DLL and your time and willing to dig into the adventure.