Why Is My GT 610 Only Running at PCIe 2.0 x8?

For a GT 610, the difference between PCIe 2.0 x8 and x16 is essentially irrelevant. The card is too slow to saturate even an x8 connection, so you probably would not notice any real-world performance loss.

The GT 730 is faster mainly because it is based on a newer generation and may have more memory, not simply because of its PCIe lane configuration. Neither card is likely to make meaningful use of the full bandwidth provided by PCIe 2.0 x16.

Some versions of the GT 610 may be physically x16 but electrically wired for fewer lanes. There were even PCI versions of the card, and performance losses from using a very limited connection are small. I would not spend time or money repairing this unless you are seeing other problems.

Check the exact specifications for your particular GT 610 model. A connector that physically fits an x16 slot does not guarantee that all sixteen lanes are electrically connected. If the card is stable and performing normally, x8 is not something that needs fixing.