140 rounds is single lv 1 fighter vs CR 23 dragon. In actual play situation, it would be 4-5 PCs, which drops it down to 35 ish rounds (or about 3.5 minutes of fight). If you bump them to lv5, it gets worse for the dragon. Point is, 1000 HP is a lot and monsters don't dish out that much damage per round. Also, AC is up to wazoo in 5e, that dragon has AC21. Also, no SR in 5e, so targeting AC works. To be fair, i used green dragon cause it was first one i found. Red ones are beefier with more hp, bit more damage and slightly higher AC. Against red one, lv 1 party would probably be wiped. Lv 5 party would have solid challenge. Frightful presence is nuisance, party would probably fail save, flee for 10 rounds, then become immune and return to kick ass.