TTS API comparison 2026
Gradium TTS vs OpenAI GPT-4o mini TTS Gradium vs OpenAI · text-to-speech for voice agents
Gradium TTS is in the Headroom speed tier (P95 at or under 101 ms); OpenAI GPT-4o mini TTS is in the Visible delay speed tier (P95 of 320 ms or more). Below, each measure side by side with its source; a model leads on a measure only beyond the tie rule. No combined score, no overall winner.
Data read October 22, 2026 · Coval, Speko, Artificial Analysis
Side by side
| Measure | Gradium TTS | OpenAI GPT-4o mini TTS |
|---|---|---|
| Speed tierCoval P95, 30 days | Headroom | Visible delay |
| Median time to first audioCoval, 30-day window | 55 ms | 950 ms |
| P95 time to first audio (slow turns)Coval, 30-day window | 101 ms | 5,055 ms |
| Time to first audio, board chartSpeko, US East | 55 ms | 518 ms |
| Naturalness EloSpeko | 1,592 | 1,424 |
| Listener Elo (95% interval)Artificial Analysis | 1,176 ±13 | not rated |
| Numbers and dates (0 to 1)Speko | 0.27 | 0.44 |
| List price per 1M charactersCoval, Speko, Artificial Analysis | $35.9 to $57.8 | $20 |
In bold: the model ahead on that measure. “tie”: latencies within 10 ms, or Elo scores whose 95% intervals overlap.
Where Gradium TTS is ahead
- →Median time to first audio: 55 ms vs 950 ms (Coval, 30-day window)
- →P95 time to first audio (slow turns): 101 ms vs 5,055 ms (Coval, 30-day window)
- →Time to first audio, board chart: 55 ms vs 518 ms (Speko, US East)
- →Naturalness Elo: 1,592 vs 1,424 (Speko)
Where OpenAI GPT-4o mini TTS is ahead
- →Numbers and dates (0 to 1): 0.44 vs 0.27 (Speko)
- →List price per 1M characters: $20 vs $35.9 to $57.8 (Coval, Speko, Artificial Analysis)
Frequently asked questions
- What is the difference between Gradium TTS and OpenAI GPT-4o mini TTS?
- Gradium TTS (Gradium) is in the Headroom speed tier (P95 at or under 101 ms); OpenAI GPT-4o mini TTS (OpenAI) is in the Visible delay speed tier (P95 of 320 ms or more). Gradium TTS leads on median time to first audio, P95 time to first audio (slow turns), time to first audio, board chart and naturalness Elo; OpenAI GPT-4o mini TTS leads on numbers and dates (0 to 1) and list price per 1M characters. No combined score is computed.
- Which is faster for real-time voice agents, Gradium TTS or OpenAI GPT-4o mini TTS?
- On Coval's 30-day window read on October 22, 2026, the median time to first audio is 55 ms for Gradium TTS and 950 ms for OpenAI GPT-4o mini TTS (Gradium TTS ahead by 895 ms); the P95, the slow turn, is 101 ms and 5,055 ms (Gradium TTS ahead by 4,954 ms). Gaps under 10 ms are ties.
- Which sounds more natural, Gradium TTS or OpenAI GPT-4o mini TTS?
- Speko naturalness: 1,592 for Gradium TTS, 1,424 for OpenAI GPT-4o mini TTS. Artificial Analysis listener Elo: 1,176 ±13 and not rated. The two sources use different listeners and can disagree.
- How are these two text-to-speech APIs compared?
- Speed first, on Coval's median and P95 time to first audio; then voice quality (Speko, Artificial Analysis), accuracy on numbers and dates (Speko) and list price. Latencies within 10 ms and Elo scores whose intervals overlap are ties. Scores from different benchmarks are never added into one number.
- Can these numbers be checked?
- Yes. Each model page lists every measurement with its source and the date of the reading, and links to the benchmark page it comes from. The rules are on the methodology page.