I've had similar to this in the past and it turned out to be corrupt xrefs/layers that xrefs were on, usually caused from file crashes, but in reality it could be a number of issues causing an entity/ies to be corrupted. In the newer versions of DataCAD there is a File Repair Tool to check, but given you're on an older version, let's first look at the xrefs. A way to see if any of your xrefs are in trouble is to save out all your xref layers as DLF's (LYR's in earlier DataCAD's) - go to the Layer menu/Export/then pick the layers that contain xrefs. Once done, open the folder with the saved DLF's and generally, each xref file size should be approx 8kb - 24kb. If there are files with higher values this would indicate it's a possible cause for what you describe.
I'm assuming your xrefs are just that and not self-xrefs, hence the drawing file size remains "normal". First, do a Drawing File Purge/LyrUtil and work from the newer file. If you have earlier drawing files prior to the one which is causing the slow loading issues, open one, then save out the DLF from the earlier file, then check the DLF file size. If it's much smaller then this may resolve the slow loading problem. Ideally, you need to find a DLF that is around the 8kb - 24kb size to know it's unaffected so you may have to keep going back through earlier drawing files. Reopen the current file, tab to or make current the xref layer which has the higher DLF export file size, and Load and Replace the xref with the older one from the earlier drawing file. You may have to edit the xref to update any layers etc. depending if there have been changes between the older import and the current one. Repeat this for all the exported xref layers with high file sizes. By keeping the original layer(s) in the drawing file, your GTV's and MSP's won't be affected. If this doesn't fix the issues, then the layer(s) themselves may be corrupted so you will need to delete the layer(s), create new ones and load the exported xref(s), then update any GTV's/MSP's etc.
Other things to check are hatching scale, especially if you've imported any DWG files, corrupt symbols, also check the RFM to see that there are no multiple copies of xrefs on top of each other, and open and save, close the xref files to check there's no issues at that end. You can also open the Utility/Drawing Info window to check the layer count to see if there are any with high values.
Maybe think about upgrading to v21 as there's a lot of improvements from v10~!