I was curious since I own both, so I ran a test to compare the two.
Converting 2000 files (40GB of FLACs) to 320K CBR MP3s took Media Center 21 minutes, while dBpoweramp did this in 13.
The test was run on a 16 thread Ryzen 1700X and on an NVMe SSD.
Media Center was significantly faster at actually setting up the conversion and gave feedback on how many files it was analyzing when I dropped them into the main window, while dBpoweramp just froze up for a couple of minutes when I selected the folder.
I think the main reason that dBpoweramp was so much faster at the conversion is that it can process 16 files at a time while Media Center is limited to 8.
CPU usage with dBpoweramp was ~90% most of the time, while Media Center was only ~50% (as you might expect when only converting 8 files on a 16 thread CPU).
Of course if you're converting form an HDD rather than an SSD it's entirely possible that you would be I/O limited in both, and being able to process 16 files rather than 8 doesn't mean anything if you only have a 2, 4, or 8 thread CPU.
If your device supports AAC, I recommend setting up QAAC as an external encoder whichever program you use, rather than MP3.
AAC is the highest quality lossy format with widespread support, and QAAC has a 'true VBR' mode which is extremely high quality.
The 'True VBR' mode produces large files though; 40-50% the size of FLAC, while something lower quality like 256k might only be be 1/3 the size of FLAC.
Some information on setting that up here:
https://yabb.jriver.com/interact/index.php/topic,113115.msg782431.html#msg782431