[v8volvo]

Glow plugs would be a good starting point. In cold weather, with a motor even in only halfway decent condition, these cars fire instantly with 6 good plugs - should start so fast that you barely have time to hear the starter run. With one dead plug it should start fast, but skip until the cyl with the dead plug lights. With two dead plugs starting is harder, and with three or more there's a good chance it will never start.

Glow plug quality matters too though. Bosch plugs are the only one to use in this engine. Champions and Autolites are easier to find but often heat more slowly, or heat in the wrong place, seen a few that heated up the base rather than the tip. They also can have a short life, especially if they run for longer designed or run with the engine running and alt charging due to a glow control issue - I remember you described some relay problems so that could have killed a few. The Bosch Duraterm plugs are self-regulating so they don't get killed as quickly by this. If you have some dead plugs that would not be a large surprise.

Compression issue is not too likely based on your description, typically with these, the first sign of a tired engine is difficult *warm* starting or smoke/oil use. However if the glow plugs don't give you progress, there are some other possibilities
- valve lash - check it, adjust if needed (cold engine only)
- injection and/or cam timing - check, adjust if needed (special tools required but available for free loan or cheap purchase on ebay, under $50)
- air leakage into fuel system - the fact that multiple glow cycles affects ease of starting for you makes this less likely, but possible. Any signs/smell of diesel fuel leakage under the hood? Does it take longer to start if parked facing uphill vs downhill?