My ex dragged his heels - in his case he was lying to ow/now 2nd wife that it was me dragging things out, he was avoiding divorce being finalised in order to avoid remarrying!
She was pregnant and pushing to marry before baby was born which didn't end up happening.
Once I found out due to a comment she made at a handover of dd, next time I saw her I handed her evidence that it was him! Based on what I saw once they were back in their car it didn't go down well 
But yes if he's with someone new it's possible it's a way of delaying getting more serious with her, or someone's in his ear about finances...
There'll be a reason you need to figure out what, possibly with help from your solicitor.
At some point from what I recall there are things your and his solicitor can do to force things to go faster.
My ex tore through solicitors anyway as he wouldn't listen to anyone! He wasn't happy about financial decisions, pissed me and dd about over contact, harassed me by phone and in person, harassed his ex solicitors...
At one point he was even almost removed from a court hearing about contact because he wouldn't quit arguing with his own solicitor during it!!
It was an education because until then I hadn't known it could be quite difficult for a solicitor to divest themselves of a client.
My own 2nd solicitor (1st was useless) was somehow able to get info on what happened, by the time I was with her he was on his 4th and by the time the divorce was complete his 5th.
It all got sorted eventually but would have been much easier/quicker if he'd just listened to someone.
Hope you get things sorted soon